Players are unable to host or join a MP game in Steam.

- You need to be friends on the Xbox app. (Just be sure that the guest is friends with the host, the guest needs to send the friend invite)
- The game host creates a lobby (character selection) in the game and waits there.
- To be able to join, you go to Multiplayer >Join online game and you will see the host's lobby. [The host doesn't even need to send any invite] 
- When everyone is in the lobby, the host can load the game (not before).
- If anyone disconnects or crashes, everyone needs to leave the game, the host needs to remake a new lobby and the others must join the same way as explained before.

PC Steam